1. Field of the Invention
The present invention relates to a gas bearing, and to a lithographic apparatus provided with such a bearing.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus contains a large number of moving parts, where a relatively large mass is moved at high speed and at a high accuracy to a specific position, possibly with a predetermined setpoint profile of the movement. The moving part is to be moved relative to a frame part, which frame part may be movable in turn relative to another frame part. As an example, reference is made to a substrate stage having a substrate support which carries a substrate. The dimensions of a substrate may be considerable, and hence the dimensions and weight of the substrate support are considerable.
In order to ensure a smooth movement of the moving part with a required change in position over time, and with a required accuracy, the moving part is supported by gas bearings relative its associated frame part. A gas bearing includes two surfaces which are highly complementary to each other. The surfaces usually are flat and straight, but they may also be curved, or e.g. V-shaped when viewed in cross-section. Between the surfaces, a gas film is maintained allowing a virtually frictionless movement of the moving part relative to the associated frame part.
In the prior art, a gas bearing assembly composed of one or more gas bearing parts connected to each other and to a magnet plate to be movable relative to a ferromagnetic frame part has been used. The magnet plate includes permanent magnets, drawing the gas bearing parts towards the frame part. Thus, a pre-tensioned or pre-loaded gas bearing is obtained. However, the surface area on the frame part needed for such a gas bearing assembly is relatively large, since it includes the combined surface area of the gas bearing parts and the magnet plate. A further drawback is the stiffness needed for the gas bearing assembly, resulting in a relatively bulky structure, adding mass to the moving part.